1. Field of the Invention
This invention relates to a display device, especially to a display device with a DA converter that converts a digital image signal to an analog image signal.
2. Description of Related Art
There has been a great demand on the market for portable communication and computing devices such as a portable TV and cellar phone. All these devices need a small, lightweight and low-energy consumption display device, and development efforts have been made accordingly.
FIG. 15 shows a circuit diagram corresponding to a display pixel element of a conventional liquid crystal display device. A plurality of the display pixel elements arranged in a matrix form configures the pixel element area in the liquid crystal display device.
A gate signal line 51 is disposed on an insulating substrate (not shown in the figure) in one direction and a drain signal line 61 is disposed in a direction perpendicular to the gate signal line 51. A pixel element selection thin film transistor 72 connected to both signal lines 51, 61 is formed near the crossing of the signal lines 51, 61. A thin film transistor will be referred to as a TFT hereinafter. A source 11s of the pixel element selection TFT 72 is connected to a pixel element electrode 80 of a liquid crystal 21.
Also, a storage capacitor 85 for holding the voltage of the pixel element electrode 80 for one field period is formed. One terminal 86 of the storage capacitor 85 is connected to the source 11s of the TFT 72, and the other terminal 87 is provided with a voltage commonly used among the pixel elements.
When a scanning signal (H level) is applied to the gate signal line 51, the pixel element selection TFT 72 turns on and an analog image signal is transmitted to the pixel element electrode 80 through the drain signal line 61 and retained in the storage capacitor 85. The image signal voltage applied to the pixel element electrode 80 is then applied to the liquid crystal 21. The liquid crystal aligns itself based on the voltage applied, forming an image in the liquid crystal display. Therefore, the liquid crystal can accommodate itself to both moving picture display and still picture display.
The analog image signal applied to the drain signal line 61 is obtained by converting an input digital image signal through an analog-digital conversion by a DA converter. The DA converter is formed near driver circuits in the peripheral area of the pixel element in the conventional liquid crystal display device having the DA converter inside a display panel.
However, since the DA converter is formed near the driver circuits in the conventional liquid crystal display device, the design of the circuits surrounding the pixel element portion becomes complicated, and increases the framing area for the display panel. Especially, when a gradation voltage is inputted from outside, the number of wiring increases in proportion to a square of the number of the gradation.
Also, the width of the area capable of accommodating the DA converter is limited, because the DA converter should be disposed corresponding to each row of the pixel elements. The maximum number of the bits of the DA converter, which can be put into this limited width, is four. Therefore, the conventional display device has a limitation in the number of the gradation.